Monolithic microchip laser with intracavity beam combining and sum frequency or difference frequency mixing

ABSTRACT

A method for producing low-noise laser output at various wavelengths and/or in various operation modes in a monolithic microchip laser comprises schemes of generating two fundamental beams in separate cavities, precise intracavity beam combination based on the walk-off effect in birefringent crystal, and wavelength conversion in nonlinear optical crystals. The fundamental beams are produced from light sources selected upon the desired wavelengths, polarizations, and other features related to the laser output. Low-noise laser devices operated in SLM or with spectra of flat-top or desired bandwidths are constructed according to the method. High-volume fabrication is feasible. Apparatus of compact size and efficient frequency conversion is demonstrated with various configurations including those for generating low-noise 491 nm laser, as a replacement of Argon ion laser.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No. 60/663,503, filed Mar. 18, 2005, and U.S. Provisional Application No. 60/708,596, filed Aug. 15, 2005, which are hereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

This invention relates generally to solid-state lasers, and in particular to monolithic microchip lasers using intracavity beam combining and sum frequency mixing (SFM) or difference frequency mixing (DFM) to generate a variety of lasing wavelengths including 457 nm, 473 nm, 491 nm, 505 nm, 532 nm, and 593 nm with low optical noise.

BACKGROUND OF THE INVENTION

Continuous-wave (CW) monochromatic lights at a number of wavelengths such as blue (λ˜490 nm) and orange (λ˜590 nm), which are useful for scientific research and medical or industry applications, cannot be directly generated from a laser diode or diode-pumped solid-state (DPSS) laser. As a consequence, traditional air-cooled Argon ion lasers, HeNe lasers, and dye lasers, though bulky and inefficient, have been the workhorse for the past years and are still playing important roles in these spectral ranges.

It is therefore highly desirable to develop compact and efficient solid-state lasers as their replacement.

In the past decade, a number of efforts have been devoted to the development of blue, green, or orange lasers by the use of indirect methods. The most commonly used method is based on frequency doubling technology. When a light of appropriate wavelength, e.g. 980 nm emanating from a laser diode, passes through a nonlinear crystal or waveguide, its second harmonic is generated due to the light-material interaction, resulting in doubled frequency, i.e., 490 nm. The frequency doubling can be realized through a direct single pass or through external cavity resonance with periodically poled nonlinear crystals or nonlinear waveguides. Lasers based on such approaches can be made extremely compact and highly efficient. In fact, some commercially available products have been developed to replace the legacy ion lasers. However, these lasers are generally expensive and may have long-term reliability issues, especially when costly waveguide materials are employed.

An alternative technology for wavelength conversion is based on Sum Frequency Mixing (SFM). As an example, in U.S. Pat. No. 5,345,457, Zenzie and Moulton demonstrated a dual-wavelength laser system with intracavity sum-frequency mixing. A Brewster prism assembly or a dichroic mirror was employed for combining the two input beams. As another example, in U.S. Pat. No. 5,802,086, Hargis and Pessot have investigated monolithic DPSS microlasers based on intracavity optical frequency mixing. However, these systems generally have shortcomings of complicated structure, high loss, and unstable operation. In addition, the achievable wavelengths in dual-wavelength lasers are limited to transitions with similar stimulated emission cross sections. For example, lasers at the orange wavelengths near 590 nm can be generated in Neodymium ion doped crystals by intracavity SFM because the ratio of the stimulated emission cross-sections for transition ⁴F_(3/2)→⁴I_(1/2) and transition ⁴F_(3/2)→⁴I_(13/2) is nearly one. In fact, both CW and Q-switched lasers at 593 nm through intracavity sum frequency mixing of 1064 nm and 1342 nm have been demonstrated by Chen et al. in publications appeared in Optics Letters Vol. 27, No. 6 and No. 20. More recently, Momiuchi et al. in U.S. Pat. No. 6,816,519 described generation of 593 nm laser through SFM of 1064 nm and 1342 nm. In their devices, the two fundamental lights were generated in separated resonant cavities with separated pumping sources and combined in a common nonlinear optical medium for wavelength conversion. The drawbacks of these devices include intracavity loss due to insertion of wavelength separating plates and stringent coating requirements.

In principle, laser output at blue spectral region near 488 nm to 492 nm can also be obtained from SFM between, e.g., well established 1064 nm and 914 nm lines. These lines can be generated from neodymium doped lasers such as Nd:YVO₄ and Nd:GdVO₄. This scheme, however, cannot be simply realized in a dual-wavelength laser because the transition ⁴F_(3/2)→⁴I_(9/2) (914 nm) is about one order of magnitude weaker than the transition ⁴F_(3/2)→⁴I_(11/2) (1064 nm). The former corresponds to a quasi-three level system, in which the lower energy level coincides with the ground electronic state. It is difficult to create and maintain population inversion in such a system when a strong transition, e.g. ⁴F_(3/2)→⁴I_(11/2), is present in the same medium. One way to resolve this problem is to use a separate high power single mode or multimode semiconductor laser as the source of 914 nm line. An example of such systems was demonstrated by Johansson et al. in Optics Express Vol. 13, No. 7. Another way to reduce the gain competition effect is adjustment of mirror reflectivities and/or alignments. Dual-wavelength lasers based on this scheme to obtain CW blue radiations were recently reported by Herault et al. in Optics Express Vol. 13, No. 15. However, their architectures are complicated, requiring many optical components, and the efficiency is low.

Other challenges for intracavity frequency doubled lasers or intracavity SFM lasers include reduction of optical noises such as amplitude fluctuations in the green output caused by nonlinear interactions of the longitudinal modes. Three solutions to the “green problem” have been investigated in the prior art: (1) lasers operated with a great many longitudinal modes (˜100) to average out intensity fluctuations in time domain; (2) single longitudinal mode (SLM) lasers to eliminate longitudinal mode coupling; (3) lasers operated at few modes with decoupled eigenstates of polarization. All of these approaches require the insertion of mode-selection elements and/or temperature stabilization or a long resonator, introducing additional complexity and cost.

It would be an advantage and, in fact, an object of the present invention as well, to provide a method whereby a variety of wavelengths that are not available from a single laser diode or a DPSS laser or an ultra-compact laser based on the intracavity SFM or second-harmonic generation (SHG) schemes described in the prior art can be obtained at low optical noise.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a method and associated apparatus and device for enabling low-noise monolithic microchip lasers, wherein intracavity beam combining and sum frequency mixing (SFM) or difference frequency mixing (DFM) are used for generation of various lasing wavelengths including those not available from the prior art.

It is another object of the present invention to provide a method and associated apparatus and device for enabling low-noise multimode or single longitudinal mode (SLM) laser output or laser output with a desired bandwidth from a monolithic microchip laser based on the intracavity beam combining and frequency conversion.

It is another object of the present invention to provide an all-solid state low optical noise blue laser in the 491 nm spectral region, as a replacement of popularly used air-cooled Argon ion laser.

It is another object of the present invention to provide an all-solid state low optical noise orange laser in the 594 nm spectral region, as a replacement of popularly used He—Ne and dye lasers.

It is yet another object of the present invention to provide a number of alternative configurations, highly flexible in accordance with specific applications, for further improving the compactness and/or efficiency of the monolithic microchip laser based on the intracavity beam combining and frequency conversion.

According to this invention, two fundamental laser beams are generated from two independent laser cavities for elimination of the green problem. These two fundamental laser beams are linearly polarized with mutually orthogonal polarizations and are combined based on the Poynting vector walk-off effect in a birefringent crystal to generate the desired wavelength by means of intracavity frequency mixing in a nonlinear optical (NLO) crystal. Cavity parameters can be separately adjusted to optimize the laser performance.

According to this invention and depending on specific needs, the two fundamental laser beams can be generated at the same side of the microchip laser or at the two opposite sides of the microchip laser and enter the NLO crystal from the two opposite surfaces. With optimized configuration, the laser compactness and/or efficiency can be further improved.

According to this invention, a concave output coupler can be used to form a half-confocal cavity. This type of monolithic or hybrid-monolithic structure is designed in order to improve beam collimation and to compensate heat-induced distortion of the focal length in high power operation. High degree of beam collimation is desired for efficient and low-noise SFM.

According to this invention, the fundamental laser beams can be generated from gain media characteristic of polarization dependent emission or of polarization independent emission. The two fundamental wavelengths can be identical or different. With these flexibilities, the selectable materials and wavelengths are greatly extended.

According to this invention, one or both of the fundamental laser beams can be generated from laser diodes and/or DPSS lasers and/or optical parametric oscillators (OPO) and/or other sources that preferably emit linearly polarized lights to form monolithic or hybrid systems. Features of the specially designed fundamental beams such as wavelength coverage, radio frequency (RF) modulation, direct modulation, and broadband spectrum can be carried into the mixed output.

According to this invention, the walk-off effect can be applied to polarization discrimination. In particular, each laser cavity supports a single polarization of predetermined direction. Inclusion of intracavity polarizers is therefore not necessary. With polarization discrimination, lasing at unwanted polarizations can be suppressed.

According to this invention, the walk-off effect can be employed for splitting one pump beam into two components with mutually orthogonal polarizations. This enables simultaneous activation of two independent gain media or of two narrowly-spaced active regions in an isotropic medium or a gain medium characteristic of polarization-dependent lasing transition from a single pump source in an extremely compact and cost-effective manner.

According to this invention, a single gain medium or two independent gain media can be pumped by a dual-emitter with narrow space. With this configuration, the monolithic microchip laser can be made extremely compact and efficient. Since, at certain walk-off angle, the required length of the birefringent crystal is proportional to the distance between the two emitters, this configuration allows of a short walk-off crystal, which reduces the requirement for beam collimation. In addition, the structure can be designed for wafer-level fabrication and tests, enabling high-volume manufacturing.

According to this invention, one or more optical element(s) can be attached, in physical contact or not, to one or each gain medium for specific applications. These optical elements can be wavelength selectors for SLM or tunable laser operation or gain compensators for broadband and/or multimode laser outputs or a set of frequency filters for laser spectra of desired bandwidth. These optical elements can also be electro-optic devices such as Pockels or Kerr cells, or acousto-optic devices or passive devices such as doped crystal characteristic of nonlinear transmission for Q-switch or mode locking.

According to this invention, crystals and optical elements are optically bonded each other to form a monolithic structure of low intracavity loss. Low-cost and high-volume fabrication is therefore feasible.

According to this invention, blue lasing at 491 nm or green lasing at 532 nm or orange lasing at 594 nm can be generated through intracavity SFM of 914 nm and 1064 nm or 1064 nm and 1064 nm or 1064 nm and 1342 nm in a microchip device. These fundamental laser beams are generated from individual laser cavities, combined within the cavities via walk-off in a birefringent crystal such as un-doped YVO₄ or TiO₂ crystal or the like, and sum frequency mixed in a nonlinear crystal such as KTP or the like. With sophisticated selection of the fundamental wavelengths and nonlinear optical processes, many other useful wavelengths can be achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention may be more completely understood by reading the following detailed description of various embodiments in connection with the accompanying drawings, in which:

FIG. 1A is a schematic illustration of a preferred embodiment of a monolithic microchip laser according to the present invention;

FIG. 1B is a schematic illustration of an alternative embodiment of a microchip laser wherein a half-confocal cavity is employed according to the present invention;

FIG. 1C is a schematic illustration of an alternative embodiment of a monolithic microchip laser wherein a concave mirror is attached to the nonlinear optical crystal to form half-confocal cavity;

FIG. 1D shows angular divergence of the Gaussian beam 175 in FIG. 1B as a function of the distance to mirror 171;

FIG. 1E is a schematic illustration of another alternative embodiment of a microchip laser wherein a folded cavity is employed according to the present invention;

FIG. 1F is a schematic illustration of another alternative embodiment of a monolithic microchip laser wherein the two fundamental waves enter the nonlinear optical crystal from the two opposite sides according to the present invention;

FIG. 2A shows a scheme for intracavity polarization control based on the walk-off effect;

FIG. 2B illustrates an application of the intracavity polarization control scheme to a dual-emitter-pumped monolithic microchip laser, in which the two fundamental laser beams are generated from an isotropic gain medium or from a gain medium characteristic of polarization-dependent radiative transitions;

FIG. 2C illustrates an application of the intracavity polarization control scheme to a fiber-laser-pumped monolithic microchip laser, in which the two fundamental laser beams are generated from an isotropic gain medium or from a gain medium characteristic of polarization-dependent radiative transitions;

FIG. 2D shows a scheme for wafer-level fabrication of the monolithic microchip lasers according to the present invention;

FIG. 3A is a flowchart of wavelength conversion for 491 nm laser;

FIG. 3B is a flowchart of wavelength conversion for 593 nm laser;

FIG. 4 shows walk-off displacement as a function of YVO₄ crystal length at 914 nm and 1342 nm;

FIG. 5A shows dependence of the 491 nm laser output power on the pump power ratio;

FIG. 5B shows the optimized pump power ratio and the corresponding 491 nm laser output power as a function of the total pump power;

FIG. 6A shows dependence of the 593 nm laser output power on the pump power ratio;

FIG. 6B shows the optimized pump power ratio and the corresponding 593 nm laser output power as a function of the total pump power;

FIG. 7A is a schematic illustration of a preferred embodiment of the inventive monolithic microchip laser operated in single longitudinal mode;

FIG. 7B shows a scheme for producing laser output with flat-top spectrum;

FIG. 8A shows a preferred embodiment of the inventive hybrid-monolithic microchip laser, in which one fundamental laser beam is generated from a laser diode;

FIG. 8B shows a preferred embodiment of the inventive monolithic microchip laser, in which one fundamental laser beam is generated from an optical parametric oscillator;

FIG. 9A shows a preferred embodiment of the inventive monolithic microchip laser, in which a birefringent crystal is integrated for splitting one pump beam into two components of orthogonal polarizations;

FIG. 9B shows an alternative embodiment of the inventive monolithic microchip laser, in which a birefringent crystal is integrated for splitting one pump beam into two components of orthogonal polarizations;

FIG. 9C shows another alternative embodiment of the inventive monolithic microchip laser, in which a birefringent crystal is integrated for splitting one pump beam into two components of orthogonal polarizations;

FIG. 9D shows yet another alternative embodiment of the inventive monolithic microchip laser, in which a birefringent crystal is integrated for splitting one pump beam into two components of orthogonal polarizations.

DETAILED DECRIPTION OF THE INVENTION

The present invention introduces a highly advantageous and heretofore unseen method and associated apparatus to enable monolithic microchip lasers that generate a variety of wavelengths, in particular, laser outputs at 457 nm, 473 nm, 491 nm, 505 nm, 532 nm, 593 nm, and 628 nm with low optical noise.

Referring now to the drawings and in particular to FIG. 1A, wherein a first embodiment of a monolithic microchip laser constructed according to the present invention is shown in a schematic form. The monolithic microchip laser 100 comprises pumping lights 115 and 125, laser gain media 160 and 170, a birefringent crystal 180, and a nonlinear optical crystal 190. These crystals are optically bonded and in physical contact with each other. Cavity mirrors 161, 171, and 191 are dielectric coatings and are respectively deposited onto the exterior surfaces of crystals 160, 170, and 190 to provide high reflectance and/or high transmittance at the desired wavelengths. With this monolithic architecture, the intrinsic passive cavity loss is minimized, while the thermal and mechanical stabilities of the cavity are improved. A high cavity Q is essential for efficient intracavity SFM or DFM, especially when a weak laser transition is involved.

For maximizing the walk-off angle ρ, the cut angle θ_(c) of the birefringent crystal 180, which is the angle between the optic axis Z and the normal to the crystal surface, is preferably π/4 or 3π/4, depending on the type of the crystal.

In laser operation, the pump light 115 activates the gain media 160 while the pump light 125 activates the gain medium 170. One laser oscillation is formed between mirrors 161 and 191 and generates a laser beam 165 having a fundamental wavelength λ₁. Another laser oscillation formed between mirrors 171 and 191, generating a laser beam 175 with another fundamental wavelength λ₂. Preferably, the polarizations of these two laser beams are mutually orthogonal with proper orientations relative to the birefringent crystal 180 to form e-ray and o-ray, respectively. The laser beams 165 and 175 are combined in NLO 190 with type II phase matching to produce a new laser beam 195 with the reduced wavelength λ₃=λ₁λ₂/(λ₁+λ₂) for SFM or increased wavelength λ₄=λ₁λ₂/|λ₁−λ₂| for DFM and polarization o if 190 is a positive crystal or polarization e if 190 is a negative crystal. The newly generated laser beam 195 is extracted through the output coupler 191.

One advantage of this configuration is that power and polarization of the pumping sources 115 and 125 can be individually adjusted to maximize pumping efficiency and optimize wavelength conversion. Another advantage comes from the fact that laser oscillations are generated in different active regions so that problems related to intensity mismatch of the mixed fundamental radiations in a dual-wavelength gain medium, as encountered in the prior art, are resolved. A further advantage is attributed to elimination of the “green problem” because the fundamental wavelengths are generated in separate cavities. Yet another advantage of this configuration is all-time low noise output whether the fundamental laser beams are in single longitudinal mode (SLM) or multiple modes (MM) or a combination of SLM and MM. Due to their independent operation, the two fundamental lasers can be aligned independently to reach optimized overlap in the nonlinear crystal. In particular, the laser with a relatively high gain and, consequently less sensitivity to the ambient, has a slightly larger beam size so that the other laser with a smaller beam size will be easily overlapped. This makes the SFM or DFM laser output more stable, while the energy loss from the edge of the larger laser beam is limited.

An alternative embodiment of the present invention is shown in FIG. 1B, wherein a concave output coupler 130 is added in order to improve the beam collimation, which reduces optical loss and noise due to frequency filtration effect in the birefringent crystal 180 and is desirable for efficient and low-noise SFM, especially when the walk-off crystal is relatively thick. In addition, the concave output coupler 130 plays a role for compensating heat-induced distortion of the focal length and resolving the optical instability problem thereby. This type of hybrid-monolithic structure is particularly useful for pump sources of high power and/or small beam size.

Half-confocal resonator can also be made in monolithic structure. As shown in FIG. 1C, a concave output coupler 139 is optically bonded onto the exterior surface of the nonlinear optical crystal 190. An additional advantage of this configuration is heat dissipation enhanced by the output coupler 139.

FIG. 1D shows the angular divergence of a diffraction-limited Gaussian beam in laser cavities with a concave mirror of various curvatures, assuming the lengths of the gain medium 170, the birefringent crystal 180, and the nonlinear optical crystal 190 are, respectively, 0.5 mm, 10 mm, and 5 mm. As the radius of curvature increases, the beam divergence decreases. The total length of the half-confocal cavity equals the curvature divided by two. In all the cases studied, high degree of beam collimation is achieved in the birefringent crystal and in the nonlinear optical crystal.

The configuration illustrated in FIG. 1B can be alternated by folded resonators for best satisfaction of the requirements for long cavities and small beam waists. FIG. 1E shows an example of such systems.

The pumping sources 115 and 125 shown in FIGS. 1A, 1B, 1C, and 1E can be generated from two separated laser diodes, or two diode submounts packaged together for compact foot print, or two diodes packaged into a dual-emitter diode package to constitute a monolithic device, or two fiber-coupled emitters, or a pump diode with a beam splitting mechanism. Each pump beam is optically coupled to its corresponding laser gain medium through a direct coupling at close distance or through a set of beam shaping lenses or though a micro-integrated lens array. In the last embodiment, the glass lens array can be optically bonded to the outer surface of the gain medium to minimize the transmission loss. The monolithic microchip device composed of optically bonded pump lens arrays, gain media, birefringent crystal, and nonlinear crystal for SFM or DFM can be fabricated in large sizes and diced into many identical devices for low cost volume production.

Another alternative embodiment of the present invention is shown in FIG. 1F. The monolithic microchip laser 104 comprises pump sources 110 and 120, laser gain media 160 and 170, a birefringent crystal 180, and a nonlinear optical crystal 190. These crystals are optically bonded and in physical contact with each other.

The first pump source 110 emits the first pump light 115 linearly polarized along the direction whereof the first gain medium 160 exhibits the strongest absorption. On another side of the monolithic microchip laser 102, the second pump source 120 emits the second pump light 125 linearly polarized along the direction favorable to the absorption of the second gain medium 170. For isotropic gain media, polarizations of the pump beams are not required. Through the beam shaping elements 112 and 122, the pump lights 115 and 125 are focused and respectively enter into their corresponding gain media 160 and 170. In order to avoid possible cross-talking, these two gain media are preferably different materials and have no overlapped emission spectra. In comparison with the configurations illustrated in FIGS. 1A, 1B, 1C, and 1E, the two-sided pump scheme is more compact under certain circumstances because it eliminates the restriction related to the finite spacing between two pump sources.

Cavity mirrors 161, 181, and 191 are dielectric coatings and are respectively deposited onto the exterior surfaces of crystals 160, 180, and 190 to provide high reflectance (HR), anti-reflectance (AR), or high transmittance (HT) at the desired wavelengths. In particular, the mirror 161 is AR to the first pump wavelength λ_(p1) and HR to the first fundamental wavelength λ₁. Similarly, the mirror 191 is AR to the second pump wavelength λ_(p2) and HR to both the first and second fundamental wavelengths λ₁ and λ₂, as well as HR to the mixed wavelength λ_(m), which is λ₃=λ₁λ₂/(λ₁+λ₂) for SFM or is λ₄=λ₁λ₂/|λ₁−λ₂| for DFM. The mirror 181 is HR to the second fundamental wavelength λ₂ and HT to the mixed wavelength λ_(m). The mirrors 161 and 191 form the first resonant cavity to support oscillation of the first fundamental laser beam 165. On the other hand, the second resonant cavity, which is composed of the mirrors 181 and 191, supports oscillation of the second fundamental laser beam 175. In order to suppress lasing from competitive wavelengths, these mirrors are also coated HT to the unwanted wavelengths.

According to our inventive teachings, the gain media 160 and 170 are so oriented that the emitted lights 165 and 175 are respectively o-ray and e-ray in the birefringent crystal 180. These two fundamental beams are collinearly combined in the nonlinear optical crystal 190, wherein the SFM or DFM takes place. Advantageously, the frequency mixing based on this configuration is a resonant process. For type II phase matching in a negative crystal 190, the beam 195 at the mixed frequency is polarized in coincidence with the polarization plane of the second fundamental beam 175. Both beams depart from the first fundamental beam 165 due to the walk-off angle.

An optional concave mirror 130 can be implemented to form a half-confocal cavity, which improves the optical stability, especially when high power and/or small beam size are involved. In addition, locating the beam waist in the NLO crystal is of benefit to spatial overlap and, therefore, the efficiency for nonlinear frequency mixing. When the concave mirror 130 is utilized, the coating 161 is AR to the first pump wavelength λ_(p1) while the interior surface of 130 is HT to the first pump wavelength and HR to the first fundamental wavelength. Optionally, another concave mirror may be added on the opposite side of the monolithic microchip laser, between the beam shaping element 122 and the NLO crystal 190, to form a confocal resonator.

According to the present invention, the walk-off effect in the birefringent crystal enables polarization discrimination in the absence of intracavity polarizers. With reference to FIG. 2A, wherein a design, which exploits directional difference between two orthogonally polarized beams after passing through a birefringent crystal, is shown. In particular, the ordinary component of the first fundamental beam, 265-o, if any, diverges away from the first cavity formed between the mirrors 261 and 291, which supports only oscillation of the extraordinary component 265-e. Similarly, the extraordinary component of the second fundamental beam, 275-e, if any, diverges away from the second cavity formed between the mirrors 271 and 291. Only the ordinary component 275-o is recycled. Insertion of an intracavity polarizer, which is required in the prior art for polarization selection, is not needed herein. After passing through the AR coating 292, the two fundamental beams 265-e and 275-o are combined in the NLO crystal 290, wherein the new wave at the sum frequency is generated.

An important application of the polarization discrimination scheme is monolithic microchip lasers, in which two fundamental laser beams are generated from a single gain medium isotropic in respect of emission polarization. As illustrated in FIG. 2B, two separated active regions in an isotropic gain medium 267 are respectively activated by pump beams 211 and 212 emanated from a dual-emitter 218. Collimation of an elongated light source such as a laser diode, which has large beam divergence along the plane perpendicular to the emitter, called fast axis, and smaller divergence along the plane of the emitter, called slow axis, can be accomplished by beam shaping optics 233, 234, and 236, which can be cylindrical macro- or micro-lenses oriented in the two orthogonal directions or a single lens made up of crossed cylindrical surfaces on each side of the lens. By matching the divergence of the fast axis with the divergence of the slow axis, pump beams with circular shape can be obtained. Since the length of the birefringent crystal 280 is proportional to the separation of the pump beams 211 and 212, and is a function of the walk-off angle, this pump scheme enables a very compact device structure. Of course, other pump sources such as two separated laser diodes or a pump diode with beam splitting mechanism can also be used.

Alternatively, the pump light sources can be two fiber-coupled emitters. As shown in FIG. 2C, the pump lights 211 and 212 extracted from the coupling fibers 219 and 229 (the emitters are not shown in this graph) are collimated and focused through beam shaping elements 235 and 245, which can be microlens arrays. With this configuration, the length of the birefringent crystal 280 can be further reduced because the separation between the two pump beams is essentially determined by the radius of the fibers, which in general are smaller than the distance between the two emitters.

As a result of optical pumping, population inversion is established in each activated region in the gain medium 267. In sharp contrast to the prior art, wherein polarization selection is accomplished by insertion of an intracavity polarizer, the inventive structures shown in FIGS. 2B and 2C select the desired polarization through the directional difference between the e-ray and o-ray in the birefringent crystal 280, which, according to our inventive teachings, also plays a role for intracavity beam combination. Due to the polarization discrimination, the resonant cavity composed of the mirrors 261 and 291 supports only oscillation of the stimulated emission linearly polarized in the principal plane of 280 to form the extraordinary beam 265-e, while the cavity between the mirrors 271 and 291 establishes resonant oscillation for the stimulated emission linearly polarized normal to the principal plane of 280 to form the ordinary beam 275-o. Frequency mixing takes place between the fundamental beams 265-e and 275-o. With this configuration, the selectable gain media and achievable wavelengths are greatly extended. In addition, the device can be made extremely compact.

An interesting example of the gain media applicable to the configurations of FIG. 2B or FIG. 2C is Nd:YAG, which exhibits polarization-insensitive emission peaks around 1064 nm and 946 nm. With the configurations shown in FIG. 2B or FIG. 2C, it is possible to produce laser output at 532 nm or 473 nm or 501 nm, depending on cavity mirror coatings.

Another important application of the polarization discrimination scheme conceptually illustrated in FIG. 2A is a monolithic microchip laser comprising gain medium or media, in which two radiative transitions of different polarization states may be obtained from same upper and lower energy levels. For example, upon excitation by a pump light to energy level ⁴F_(3/2), the extraordinary transition between ⁴F_(3/2) and ⁴I_(11/2) corresponding to wavelength of 1047 nm (π) and the ordinary transition between the same energy levels corresponding to wavelength of 1053 nm (σ) may occur simultaneously in Nd:YLF, with nearly equivalent stimulated emission cross-sections. Fundamental wavelengths are determined by the transition lines in the gain medium, the polarization states of the emission, the coating characteristics, and the different indices of refraction of the e-ray and o-ray in the birefringent crystal. Undesired stimulated emission may deplete the population of the upper state, which reduces the laser efficiency and even disturbs the frequency mixing process. In the configurations displayed in FIGS. 2B and 2C, lasing at undesired wavelength can be suppressed by orienting its polarization along unfavorable direction of the birefringent crystal 280. Advantageously, the fundamental beams are both polarized with extremely high purity.

By replacing the gain medium 280 in FIG. 2B or 2C with Nd:YLF, laser output at 525 nm (mixed from the π line 1047 nm and the σ line 1053 nm) or 658 nm (mixed from the π line 1321 nm and the σ line 1313 nm) or 583 nm (mixed from the π line 1047 nm and the σ line 1313 nm) or 586 nm (mixed from the π line 1321 nm and the σ line 1053 nm) can be produced.

Using a single gain medium has a further advantage: making high volume fabrication feasible. As shown in FIG. 2D, the gain medium, the birefringent crystal, and the nonlinear optical crystal can be grown into layers with uniform coatings at appropriate wavelengths, which may be broadband. In some applications, microlens arrays can be deposited on the substrate. The integrated crystal is then cut into small pieces, each forms a microchip laser. This scheme greatly improves the productivity and reduces the cost.

Our inventive teachings can be better understood through investigation of the following examples.

EXAMPLE 1 Monolithic Microchip Laser for Generating 491.7 nm Light

Returning to FIG. 1A. In this specific example, the monolithic microchip laser 100 consists of two independent pumping sources 115 and 125, two Nd:YVO₄ laser gain media 160 and 170, a birefringent crystal such as un-doped YVO₄ or TiO₂ 180, and a nonlinear crystal KTP 190. These crystals are optically bonded and in physical contact for elimination of the boundary optical loss in generation of intracavity SFM or DFM.

In a preferred embodiment, the pumping lights 115 and 125 are output from a build-in dual-emitter diode laser that emits two individual laser beams, but can also from two separate emitter diode lasers, depending on specific applications. Each emitter can have a 1 μm×50 μm aperture and emit upto 500 mW power output at 808 nm or have 1 μm×200 μm aperture and emit upto 2 W power output at 808 nm or other aperture size to emit an appropriate power. These two emitters are separated each other with a preferable distance of 500 μm, which corresponds to ˜5 mm length of the birefringent crystal 180, or other distances that match the walk-off displacement upon propagation through 180 of a given length. The pump light 115 activates the gain medium 160 and the light 125 activates 170. Preferably, these pumping beams are polarized along the π direction of their corresponding gain medium for best absorption efficiency.

The laser gain media 160 and 170 each are 1 mm long with 1% Nd-doped concentration or 0.5 mm long with 4% Nd-doped concentration or other appropriate combinations of length and Nd-doped concentration, and are a-cut to produce linearly polarized laser beam along the π direction. Preferably, the C-axes of laser gain media 160 and 170 are orthogonal to each other so that the laser beam generated from 160 is polarized in the principal plane of the birefringent crystal 180, known as e-beam, whereas the beam from 170 has the polarization normal to the principal plane, called o-beam.

One side of the gain medium 160 is a coating 161, highly reflective (HR) at 914 nm and highly transmissive (HT, T>95%) at 808 nm. Similarly, the exterior side of the gain medium 170 is coated HR at 1064 nm and HT (T>95%) at 808 nm. The coating 161 should also be HT at 1064 nm and 1342 nm to prevent these high-gain transitions from lasing. The coating 191 on another side of microchip 100 is HT at 491.7 nm (T>95%), and HR at both 914 and 1064 nm.

In operation, the gain media 160 and 170 are longitudinally pumped by the pumping sources 115 and 125. Upon excitation from the ground state ⁴I_(9/2) to the metastable state ⁴F_(3/2) by the pumping sources, laser gain media 160 and 170 respectively emit 914 nm and 1064 nm beams. Within the birefringent crystal 180, the 914 nm beam is e polarized, whereas the 1064 nm laser beam is o polarized. Owing to the walk-off effect, these two beams are combined at the exit of the birefringent crystal 180. With precise control of the un-doped YVO₄ or TiO₂ crystal length, the two fundamental beams collinearly enter the nonlinear optical crystal KTP 190 and frequency-mixed therein. In some applications, configurations illustrated in FIG. 1B or 1C or 1E or 1F can be used. FIG. 3A shows a flowchart of the wavelength conversion.

Advantageously, an algorithm for automatic power control enables individual adjustment of pumping powers for optimizing SFM efficiency. Still advantageously, a single thermoelectric controller (TEC) is employed for automatic temperature control of both laser diodes and the monolithic device. Alternatively, the TEC can be removed for low power consumption devices.

EXAMPLE 2 Monolithic Microchip Laser for Generating 593.5 nm Light

Continuing in reference to FIG. 1A. With modification of some parameters, the same configuration can be applied to other wavelength conversion devices, e.g., 593.5 nm lasers. These parameters include proper selection of mirror coatings and KTP phase matching angle.

In order to generate laser output at 593.5 nm, the coating 161 should be highly reflective at 1064 nm and highly transmissive (T>95%) at 808 nm. Similarly, the mirror 171 is coated HR at 1342 nm and HT at 808 nm. The coating 171 is also HT at 1064 nm to prevent this high-gain transition from lasing. The coating 191 on the exterior surface of microchip 100 is HT at 593.5 nm, and HR at both 1342 and 1064 nm. In some applications, configurations illustrated in FIG. 1B or 1C or 1E or 1F can be used.

FIG. 3B shows a flowchart of the wavelength conversion. Advantageously, an algorithm for automatic power control enables individual adjustment of pumping powers for optimizing SFM efficiency. Still advantageously, a single thermoelectric controller (TEC) is employed for automatic temperature control of both laser diodes and the monolithic device. Alternatively, the TEC can be removed for low power consumption devices.

It should be noted that the above examples are for description only and by no means of limitations of our inventive teachings. It should also be recognized by those skilled in the art that selection of the crystal materials, the doped ions, the coatings, and geometries of microchip cavities is a matter of engineering design.

For further understanding of our inventive teachings and design principles, some numerical analysis is given.

Analysis 1: Pump Beam Separation and Birefringent Crystal Length

Due to the walk-off effect, an extraordinary (e) wave passing through a birefringent crystal is redirected by a walk-off angle ρ, while an ordinary (o) wave is not. If these two beams coincide with each other before entering a birefringent crystal and the crystal length is l, then a separation d=l tan ρ is observed at the output surface of the crystal.

In reference to our inventive structure shown in FIG. 1A, the displacement of the e-beam propagation direction d determines the optimal separation of the pump beams. In order to overlap o-beam and e-beam in the nonlinear crystal 190, one can either adjust the displacement d for a given birefringent crystal 180, or adjust the crystal length l for a given pump beam separation. FIG. 4 shows the dependence of these two parameters for an un-doped YVO₄ at 914 nm and 1342 nm, assuming the angle between the wave propagation direction and the optic axis of the birefringent crystal θ=π/4. Interestingly, the walk-off displacement is insensitive to the wavelength. This is an advantage for the microchip design.

Analysis 2: SFM Power and Pump Power

According to the present invention, the fundamental laser beams are generated in separate resonant cavities and different gain media activated by individual pumping sources. This allows individual adjustment of operation parameters for each laser to optimize the SFM efficiency.

With reference to the foregoing Example 1, wherein a 491 nm laser output is obtained by mixing two fundamental beams, respectively, at 914 nm, which is generated from the transition ⁴F_(3/2)→⁴I_(9/2), and 1064 nm, which is generated from the transition ⁴F_(3/2)→⁴I_(11/2). In a Nd:YVO₄ gain medium, the stimulated emission cross section for the transition ⁴F_(3/2)→⁴I_(9/2) is about an order of magnitude smaller than that for the transition ⁴F_(3/2)→⁴I_(11/2). To compensate this difference, the laser gain medium for generation of 914 nm light should be pumped by a larger power.

Plotted in FIG. 5A are curves showing the dependence of 491 nm laser output power on the pump ratio P_(914nm-in)/P_(1064nm-in) at total pump power of P_(in)=P_(914nm-in)+P_(1064nm-in)=1 W and 2 W. The curve peak identifies optimized pump ratio at the given P_(in). As the pump ratio increases from the threshold, the SFM efficiency increases rapidly. After reaching its optimal value, it decreases slowly, especially if the total pump power is low.

FIG. 5B shows the dependence of the optimized pump ratio on the total pump power. The monotonically decreasing trend to its asymptote P_(914nm-in)/P_(1064nm-in)→1 as the total pump power P_(in)→∞ indicates that optimization of the pump intensity ratio is particularly useful for relatively low power pumping. Also shown in FIG. 5B is the optimized 491 nm output as a function of the total pump power. In particular, nearly 80 mW of 491 nm laser output can be obtained by the use of total pump power of 3 W. This makes replacement of Argon ion laser with the microchip laser possible. Based on these relationships, an algorithm for automatic power control, which ensures optimized SFM efficiency all the time, can be developed.

Similarly, optimized SFM laser output at 594 nm can be obtained by individually adjusting the pump powers for the two fundamental lines. A quantitative dependence of the 594 nm laser output power on the pump ratio P_(1342nm-in)/P_(1064nm-in) at total pump power of P_(in)=P_(1342nm-in)+P_(1064nm-in)=0.5 W or 1 W is shown in FIG. 6A. In comparison with the curves for 491 nm, which are shown in FIG. 5A, the SFM efficiency for 594 nm appears more sensitive to the pump ratio at the same total pumping rate.

FIG. 6B shows the optimized pump ratio and the corresponding SFM output as functions of the total pump power. With optimized partition of 1.2 W pump power into Pump 1 and Pump 2 shown in FIG. 3B, it is possible to obtain 100 mW 594 nm laser output. Although 594 nm laser output can also be obtained through intracavity SFM using the methods disclosed in the prior art, the present invention greatly improves the efficiency.

Analysis 3: Generation of Other Visible Wavelengths

With the present invention, lasers at various visible wavelengths can be generated through selection of appropriate laser transitions, doped ions, and crystals. This is an important advantage of the present invention because many of these wavelengths are unavailable from a direct SHG or SFM based on the prior art. Tables 1A-1C show some examples, which are by no means a complete list. It should be pointed out that the emission spectra of the Yb-doped crystals are broadband. Therefore, the wavelengths presented in Table 1C are tunable.

Analysis 4: Generation of IR Wavelengths

With the present invention, lasers at various IR wavelengths can be generated through selection of appropriate laser transitions and DFM from proper nonlinear crystals. This is an important advantage of the present invention because many of these wavelengths are unavailable from a direct laser generation. Tables 1A and 1B show some examples, which are by no means a complete list.

As will become clearer from further descriptions hereinafter, our invention can be embodied and configured in a variety of ways according to different applications. With this flexibility, many additional advantages of the present invention can be demonstrated.

§1. With attachment of one or more optical elements of appropriate properties on one or each gain medium, a number of optional features can be obtained.

As a first example, attaching a wavelength selection element such as a Lyot birefringence filter or an intracavity etalon enables the monolithic microchip laser to oscillate on a single longitudinal mode with low intracavity loss and noise. As shown in FIG. 7A, a wavelength selection element 751 is sandwiched in between the gain medium 760 and the birefringent crystal 780 so that the fundamental beam 765 is SLM. Similarly, a wavelength selection element 752 is inserted between the gain medium 770 and the birefringent crystal 780, which leads to single longitudinal mode of the second fundamental beam 775. Therefore, the mixed laser output 795 is also SLM.

It should be pointed out that there may be many alternative configurations to achieve SLM operation. For example, as a modification of the configuration shown in FIG. 1E, the wavelength selection elements 751 and 752 can be respectively inserted between the gain medium 160 and the birefringent crystal 180 and between the gain medium 170 and the birefringent crystal 180. In this way, the SLM monolithic microchip laser can be made very compactly.

By adding a birefringent crystal layer to the structure illustrated in FIG. 2D for wavelength selection, high-volume fabrication of SLM microchip lasers becomes feasible.

As a second example, by attaching an off-tuned etalon for gain compensation, broadband and multimode laser output can be achieved from the monolithic microchip laser. Conventionally, etalons are inserted into laser resonators for wavelength selection. In contrast to these applications, tuning a minimum in the transmission curve of an etalon (dotted oscillating curve) in coincidence with the peak of the gain profile (dotted Gaussian curve), as graphically illustrated in FIG. 7B, results in a flat-top spectrum of laser output, which can be applied to laser optical scanning systems, optical memory devices, laser raster printing systems, laser display systems, inspection systems, lithographic systems, imaging instrumentation, and other applications where speckle reduction is necessary. Flat-top spectral output can also be extracted from resonators that are formed with specially coated mirrors for gain flattening.

Other methods for laser speckle reduction includes intentional introduction of optical phase distortions. This can be practically realized by implementation of chirped dielectric mirrors of positive dispersive properties for enhancing phase shift difference involved in reflection of various oscillation modes.

As a third example, a set of frequency filters such as Lyot filters can be attached to obtain laser output of desired spectral bandwidth.

As a fourth example, the attached optical elements can be electro-optic devices such as Pockels or Kerr cells, which produce electrically controlled birefringence for cavity Q transition, or acousto-optic devices, whose transparency is sensitive to the passage of ultrasonic wave, or a doped crystal characteristic of fluence-dependent transmission for generation of high pulse power through Q-switching or mode lock. With this type of configurations, dual Q-switched laser is achievable from the monolithic structure. When the Q-switches on the two arms are controlled independently, two sequential pulses with an adjustable time delay can be produced. Such laser systems are especially useful for time-of-flight applications.

§2. One fundamental laser beam can be produced from sources such as laser diodes, or other external laser systems with or without harmonic generations, or optical parametric oscillators (OPO) in replacement of the internal gain medium emission. With this flexibility, the available wavelength range can be further extended. Moreover, specially designed features incorporated into these sources can be carried over.

One example is graphically illustrated in FIG. 8A. In this monolithic microchip laser 801, the second gain medium is removed. As a replacement, a laser diode 820 is employed for producing the second fundamental beam 875, which, in this particular configuration, is an o-ray. As shown in this graph, the laser diode 820 is controlled by a drive circuit 825, which can be RF modulated for stabilizing the laser operation. With RF modulation, the laser diode 820 is forced to operate at multiple modes. Noise associated with mode hop and/or mode partition, as well as speckle noise due to interference pattern resulted from reflection or transmission of highly coherent light from optically rough surfaces can be eliminated or reduced. Alternatively, the fundamental beam emitted from the laser diode 820 can be characterized as an e-ray, provided that the gain medium 860 emits another fundamental beam, which behaves ordinarily (o-ray) in the birefringent crystal 880.

Another example is illustrated in FIG. 8B, where one fundamental beam is generated through optical parametric oscillation, which is the reverse process of sum frequency generation. By introducing a pump beam, a signal wave and an idler wave appear in the birefringent crystal 880. The specific wavelengths of the signal and the idler, λ_(s) and λ_(i), are determined by the phase matching conditions and the inclination of the cutting angle of the nonlinear optical crystal 850, in which the optical parametric oscillation takes place. The pump beam 811 can be generated by a laser diode or a diode pumped solid-state laser or any other laser device that produces a linearly polarized light with appropriate wavelength.

For type II phase matching, one of the output beams involved in the OPO is polarized along the principal plane of the birefringent crystal 880, known as e-ray, and the other has a polarization normal to the principal plane, known as o-ray. For the configuration shown in FIG. 8B, the e-ray, 865-e, walks off the propagation direction and combines with another fundamental beam 875, emitted from the gain medium 870, in the NLO crystal 890, wherein the sum frequency is generated. On the other hand, the o-component, 865-o, diverges away from the resonant cavity. An advantage of this configuration is elimination of possible interference between the signal and the idler.

In an alternative configuration, the fundamental laser beam 875 is polarized as an e-ray in the birefringent crystal 880, which, under the phase matching conditions, combines with another fundamental beam, the o component of the OPO output, 865-o, for the nonlinear sum frequency mixing process. In this case, the e component, 865-e, diverges away from the resonant cavity as a loss.

Although the optical parametric oscillation demonstrated in FIG. 8B is for type II phase matching, this should not be understood as a restriction of our invention. As a matter of fact, optical parametric oscillation that fulfills type I phase matching can also be utilized as a source for generation of the fundamental beam. In this case, the signal and the idler have the same propagation direction and can be distinguished by their wavelengths, more specifically, by proper coating of the resonant cavity mirrors, so that their reflectivity is peaked around the desired fundamental wavelength while is low at the unwanted wavelength to introduce loss.

Tunability is a fundamental characteristic of all parametric devices. Therefore, with the aid of a mechanism for change of the phase matching conditions, the configuration demonstrated in FIG. 8B enables achievement of wavelengths tunable over a range. Phase matching conditions can be changed by making use of the angular dependence of the birefringence of anisotropic crystals, or by temperature variation, or by electro-optic variation of the refractive indices.

An important application of the configurations described in FIGS. 8A and 8B is to produce red light. For example, wavelength 628 nm can be generated by sum frequency mixing of 1064 nm and 1530 nm. The fundamental wavelength 1064 nm can be directly obtained from stimulated emission of Nd:YAG or Nd:YVO₄. Another fundamental wavelength 1530 nm can be extracted from a laser diode or the signal wave of a KTP OPO pumped by 1064 nm.

§3. A birefringent crystal can be employed for splitting one pump beam into two components of orthogonal polarizations based on the walk-off effect.

For illustration, a preferred embodiment is hereby displayed in FIG. 9A. A laser diode 910 emits a pump beam 950, which passes through the beam shaping optics 920 for collimation and enters a birefringent crystal 930. In accordance with our inventive teachings, the pump beam 950 is linearly polarized with a predefined direction. Due to the walk-off effect, it is split into two components in the birefringent crystal 930. The component 950-e, which is polarized along the principal plane of 930, is an e-ray and its propagation direction tilts upwards by the walk-off angle. Another component, 950-o, whose polarization is normal to the principal plane of 930, is an o-ray and, therefore, remains its propagation direction. These two components are then directed into their corresponding gain media 960 and 970 as pump sources. With this configuration, a single laser diode can be employed for simultaneously pumping two gain media. According to our inventive teachings, the gain media 960 and 970 are so oriented that their preferable absorption directions match the polarizations of the pump beams directed to them. The intensity ratio of the beam component 950-e to the component 950-o is determined by the polarization state of 950. By adjusting the polarization direction of beam 950, the efficiencies for pump and frequency mixing can be optimized. This process can be accomplished by rotating the pump source, which in this FIG. 9A, is a free-space diode.

Since d_(p), the separation of the points, where the pump beams 950-e and 950-o enter their gain media 960 and 970, is determined by the length of the birefringent crystal 930, l_(p), and the characteristics thereof, and is generally small, the required length, l_(b), for the beam combing birefringent crystal 980 is short, which minimizes intracavity optical loss and makes the structure very compact. If the birefringent crystals 930 and 980 are of same type, their optic axes are mutually orthogonal. If one crystal is positive while the other is negative, their optic axes should be parallel to each other.

An alternative embodiment of the compact pump source is shown in FIG. 9B. Instead of physical contact with the gain media 960 and 970 as shown in FIG. 9A, the birefringent crystal 930 is now a separated element and is optically connected to the gain media through focus lenses 941 and 942. This embodiment is differentiated from the one shown in FIG. 9A also in respect of the beam sizes passing through the birefringent crystal 930. In this alternative embodiment, the beam 950 is expanded after passing through the telescope 921. As a consequence, the split beams 950-e and 950-o are both wide collimated in 930. This embodiment allows implementation of high pump powers.

Alternatively, the focus lenses 941 and 942 can be replaced with microlens arrays. Microlens arrays including GRIN (gradient index) lens arrays can be fabricated by processing lithography and etching on proper films deposited on substrates, which, in the configuration of FIG. 9C, are the gain media 960 and 970. In this FIG. 9C, the microlens array 945 is utilized as the beam focus element, which results in a compact pump unit.

FIG. 9D shows another alternative embodiment of the compact pump source. In particular, the free-space laser diode 910 is replaced by a fiber-coupled emitter 912. Polarization is randomized through the fiber, which results in two pump components 950-o and 950-e of equivalent intensities. This embodiment is especially useful for SFM or DFM requiring two identical pump intensities for optimized performance such as those when two fundamental beams are generated from same type of gain media for producing identical wavelengths.

The pump beam splitting scheme illustrated in FIGS. 9A-9D can also be utilized for simultaneously injecting pump energy into two separate active regions of an isotropic gain medium or a gain medium characteristic of polarization-dependent lasing transition in order to generate two fundamental beams. For these applications, the isotropic gain medium or the gain medium characteristic of polarization-dependent lasing transition is sandwiched in between two birefringent crystals: one for pump beam splitting, while the other for polarization discrimination and fundamental beam combination. This enables compact and low-cost monolithic microchip lasers. Furthermore, high-volume fabrication can be made by adding the birefringent crystal 930, as a layer, onto the structure illustrated in FIG. 2D. TABLE 1A Sum Dif Lasing Fundamental Fundamental Frequency Frequency Medium λ₁ (nm) λ₂ (nm) λ₃ (nm) λ₄ (nm) Nd:YVO₄ 1342.0 1342.0 671.0 1064.3 1342.0 593.6 5143.3 1064.3 1064.3 532.2 1064.3 914.0 491.7 6472.2 1342.0 914.0 543.7 2865.9 914.0 914.0 457.0 Nd:GdVO₄ 1341.3 1341.3 670.7 1063.1 1341.3 593.1 5125.6 1063.1 1063.1 531.6 1063.1 912.6 491.1 6446.4 1341.3 912.6 543.1 2855.3 912.6 912.6 456.3 Nd:KG(WO₄)₂ 1351.0 1351.0 675.5 1067.0 1351.0 596.2 5075.8 1067.0 1067.0 533.5 1067.0 911.0 491.4 6231.0 1351.0 911.0 544.1 2797.2 911.0 911.0 455.5

TABLE 1B Sum Dif Lasing Fundamental Fundamental Frequency Frequency Medium λ₁ (nm) λ₂ (nm) λ₃ (nm) λ₄ (nm) Nd:YAG 1318.8 1318.8 659.4 1064.2 1318.8 588.9 5512.4 1064.2 1064.2 532.1 1064.2 946 500.8 8517.2 1318.2 946 550.8 3350.4 946 946 473.0 Nd:YLF 1321 1321 660.5 1321 1313 658.5 1321 1053 585.9 5190.3 1321 1047 584.1 5047.8 1313 1313 656.5 1313 1053 584.4 5317.7 1313 1047 582.5 5168.1 1053 1053 526.5 1053 1047 525.0 1047 1047 523.5

TABLE 1C Lasing Fundamental Fundamental Sum Frequency Medium λ₁ (nm) λ₂ (nm) λ₃ (nm) Yb:YVO₄ 1010.0 1010.0 505.0 Yb:KGW 1023.0 1023.0 511.5 Yb:KYW 1025.0 1025.0 512.5 Yb:YAG 1030.0 1030.0 515.0 1030.0 1050.0 520.0 1050.0 1050.0 525.0 

1. A method for producing low-noise laser output at various wavelengths and/or in various operation modes in a monolithic microchip laser comprising schemes of: (1) generation of incoming beams for nonlinear frequency conversion: wherein (a) two incoming beams oscillated at fundamental wavelengths for frequency mixing are called fundamental beams, (b) the two fundamental beams are generated in individual cavities to eliminate the “green problem”, (c) said two fundamental beams are linearly polarized with mutually orthogonal polarizations; (2) beam combination and/or separation via walk-off effect in birefringent crystal; (3) frequency conversion such as sum frequency mixing, difference frequency mixing, harmonic generation, and optical parametric oscillation in one or more nonlinear optical crystals; (4) monolithic structure of microchip laser composed of optically bonded crystals; (5) proper selection of gain media and laser transitions involved, nonlinear optical materials and nonlinear optical processes therein, and other optical elements, depending on the required fundamental wavelengths, the wavelength to be generated, and operation modes; and (6) feasibility of high-volume fabrication.
 2. A method as of claim 1, wherein: one or more birefringent crystals are employed for polarization discrimination through walk-off to achieve intracavity beam combination or separation, wherein: beam combination is for frequency mixing of two incoming waves in a nonlinear optical crystal; beam separation is for deviating unwanted beams away from resonator cavities and directing wanted laser beams into appropriate cavities for resonant oscillation or for suppressing stimulated emissions of unfavorable polarization or for simultaneous pump of two gain media or of two separated active regions in an isotropic medium or a medium characteristic of polarization-dependent emission from a single light source.
 3. A method as of claim 1, wherein: incoming beams for nonlinear optical processes are generated from one or more of the following sources: internal gain media, laser diodes, other external lasers including but not limited to diode pumped solid-state lasers and fiber lasers, optical parametric oscillators or amplifiers.
 4. A method as of claim 3, wherein generation of a fundamental beam via an internal gain medium comprising steps of: selecting a gain medium according to the required fundamental beam; generating a pump beam that matches the absorption characteristics of the gain medium; activation of said gain medium for creating population inversion in correspondence with the radiative transition of the fundamental wavelength; circulating the stimulated emission in the corresponding cavity for resonant oscillation; suppressing unwanted radiations from lasing by means of introducing cavity losses and/or deviating said radiations from resonators based on walk-off effect; directing the generated fundamental beam to a nonlinear optical crystal for frequency conversion through a birefringent crystal.
 5. A method as of claim 3, wherein generation of a fundamental beam via an optical parametric oscillator or amplifier comprising steps of: selecting an optically nonlinear crystal according to the required fundamental beam; cutting the optically nonlinear crystal with proper inclination angle to fulfill the phase matching conditions for the desired signal and idler wavelengths and polarizations; generating a pump beam, whose wavelength and polarization satisfy the phase matching conditions for the optical parametric oscillation; directing the pump beam into the optically nonlinear crystal for optical parametric oscillation; generating the signal wave and the idler wave; selecting the fundamental beam from either the signal wave or the idler wave, depending on the wavelength and polarization requirements; directing the generated fundamental beam to another nonlinear optical crystal for frequency conversion through a birefringent crystal.
 6. A method as of claim 3, wherein generation of a fundamental beam via a laser diode comprising steps of: generating laser drive current via a drive circuit; said drive current may be modulated with high frequency such as radio frequency; rotating the laser diode to a predetermined direction such that the emitted laser beam has desired polarization relative to the walk-off birefringent crystal; directing the generated fundamental beam to a nonlinear optical crystal for frequency conversion through a birefringent crystal.
 7. A method as of claim 1, wherein one or two fundamental waves can be modified via insertion of optical elements such as: wavelength selection elements for single longitudinal mode operation; or off-tuned etalons for gain-flattening; or set of frequency filters for output spectrum of desired bandwidth; or electro-optic devices or acousto-optic devices or passive devices such as doped crystal characteristic of nonlinear transmission for Q-switch or mode lock.
 8. A method as of claim 1, comprising steps of: generation of an extraordinary (e) laser beam with one fundamental wavelength; generation of an ordinary (O) laser beam with another fundamental wavelength; walk-off of the e-beam in a birefringent crystal; combination of the o-beam and the e-beam in a nonlinear optical crystal with type II phase matching; wavelength conversion via interactions of the incoming waves with the nonlinear optical crystal; extracting the produced laser beam through output coupler(s).
 9. A method as of claim 1 further comprising steps of producing low optical noise output via SFM or DFM from two independent laser beams with any one of the following combinations: single longitudinal mode and single longitudinal mode, single longitudinal mode and multiple modes, multiple modes and multiple modes.
 10. A method as of claim 1 further comprising: steps of selecting parameters for precise intracavity beam combination and overlapping; steps of collimating two fundamental beams in the nonlinear optical crystal and reducing sensitivity of beam alignment stability; steps of selecting powers and polarization directions of pumping lights for optimal optical coupling and for optimizing SFM or DFM efficiency; a scheme employing half-confocal cavity for reducing frequency-filtration-induced optical loss in birefringent crystal; a scheme for high-volume fabrication further comprising steps: (a) crystals are grown into layers with uniform coatings at appropriate wavelengths, (b) cut the integrated crystal into small pieces, each forms a microchip laser; an algorithm for automatic power control and SFM or DFM optimization.
 11. A monolithic microchip laser that produces low noise laser output at a variety of wavelengths, with single or multiple longitudinal modes, from a compact and low-cost structure, wherein: (1) reduction of optical noise is achieved from one or more of the followings: (a) generation of fundamental beams in independent resonant cavities to eliminate the “green problem”, (b) suppression of unwanted lasing by introducing cavity losses at unwanted polarizations through walk-off induced deviation from the resonators and/or at unwanted wavelengths through wavelength-dependent reflectivities of cavity mirrors, (c) reduction of frequency filtration associated with beam divergence in birefringent crystal by shortening the separation between two pump lights, (d) stabilization of laser diode operation by implementing radio frequency modulated drive current, (e) broadband and multimode operation for averaging off noises due to mode hops and/or mode partition, and for speckle reduction, (f) reduction of frequency filtration associated with beam divergence in birefringent crystal by incorporation of a concave mirror of appropriate curvature; (2) production of laser output at a variety of wavelengths is achieved by means of (a) selection of light sources for producing fundamental wavelengths and nonlinear optical processes according to the desired output wavelength, (b) generating incoming beams and directing these beams to the nonlinear optical crystal through a birefringent crystal, (c) frequency mixing in said nonlinear optical crystal, (d) optional wavelength tunability; (3) high laser operation efficiency is achieved through (a) optimized intensity ratio of the two pump sources, (b) well-collimated beams in the walk-off crystal; (c) spatial overlap of the two fundamental beams in the nonlinear optical crystal, (d) elimination of intracavity polarizers, (e) optically bonded crystals with minimum interfacial optical loss; (4) single longitudinal laser operation can be achieved by insertion of wavelength selection elements such as Lyot filters and/or etalons; (5) flat-top spectrum of laser output can be achieved by insertion of off-tuned etalons and/or reflection from specially coated mirrors for gain compensation; (6) laser output spectrum of specified bandwidth can be achieved by insertion of a set of Lyot filters with proper transmission bands; (7) compact and low-cost structure is achieved by means of one or more of the followings: (a) monolithic structure of optically bonded crystals, (b) dual-emitter pump diode, (c) fiber-coupled emitters as pump sources, (d) schemes that employ a single laser source for simultaneous pump of two gain media characteristic of orthogonal polarizations, (e) schemes that employ a single laser source for simultaneous pump of two separated active regions of an isotropic gain medium for generation of two fundamental beams of orthogonal polarizations, (f) schemes that employ a single laser source for simultaneous pump of two separated active regions of a gain medium characteristic of polarization-dependent emission for generation of two fundamental beams of orthogonal polarizations, (g) generating one fundamental beam from one side of the microchip laser while the second fundamental beam from another side of the microchip laser and injecting the generated beams into the nonlinear optical crystal from both sides, (h) application of birefringent crystals to beam combinations and/or separations for directing laser beams to desired locations according to their polarizations, (i) utilizing microlens arrays for beam focusing/shaping, (j) utilizing a concave mirror physically attached to the nonlinear optical crystal for formation of half-confocal cavity, (k) monolithic structure capable of volume production.
 12. A monolithic microchip laser as of claim 11, comprising: at least two light sources to produce two fundamental laser beams linearly polarized with mutually orthogonal polarizations; at least one birefringent crystal, in which one of the fundamental laser beams is an e-ray and the other beam is an o-ray, wherein the e-ray is tilted via walk-off effect and the length of said birefringent crystal is so chosen that the two fundamental laser beams are combined at the exit; at least one nonlinear optical crystal for frequency conversion; other optical elements as optional attachments.
 13. A monolithic microchip laser as of claim 12, wherein: said light sources for producing fundamental laser beams are preferably selected from the group including internal gain media, laser diodes, external solid-state lasers pumped by diodes or other sources, fiber lasers, optical parametric oscillators or amplifiers, and other sources that produce linearly polarized laser beams at the desired wavelengths; sophisticated selections of incoming beams for nonlinear optical processes enable laser radiation at a desired wavelength with desired features; at least one fundamental laser beam is generated from internal gain media, wherein: each gain medium is excited by a pump beam with spatial overlap and matching with the absorption spectrum in frequency and polarization; said pump beam is optically coupled to its corresponding gain medium through: a direct coupling at close distance; or a set of beam shaping lenses; or a micro-integrated lens array.
 14. A monolithic microchip laser as of claim 12, wherein: two gain media are optically bonded at the same side of the monolithic microchip laser and simultaneously pumped by two pump beams; said two pump beams can be emitted from two separate light sources or from a dual-emitter or split from one beam emitted from a single light source, wherein: pump beam separation is achieved via walk-off of the extraordinary component in a beam-displacement birefringent crystal, integrated into the monolithic microchip laser; stimulated emissions from said two gain media oscillate in their respective resonant cavities for producing laser beams at fundamental wavelengths; said two fundamental laser beams are combined in the optically nonlinear frequency mixing crystal due to walk-off effect in a beam-combining birefringent crystal; walk-off effect in the birefringent crystal guarantees laser oscillations of different beams in different cavities, which eliminates the “green problem”.
 15. A monolithic microchip laser as of claim 12, wherein: two gain media are optically bonded at two opposite sides of the monolithic microchip laser and simultaneously pumped by two pump beams; stimulated emissions from said two gain media oscillate in their respective resonant cavities for producing laser beams at fundamental wavelengths; said two fundamental laser beams are injected into the nonlinear optical crystal from two opposite sides and combined therein for frequency mixing; walk-off effect in the birefringent crystal guarantees laser oscillations of different beams in different cavities, which eliminates the “green problem”.
 16. A monolithic microchip laser as of claim 12, wherein: a single gain medium isotropic in respect of emission polarization is energized by two separated pump beams to establish population inversion over two separated volumes; polarization of stimulated emission in each volume is selected by the resonant cavity; propagation of laser beam is directed into corresponding resonant cavity through walk-off effect; each resonant cavity supports oscillation of one fundamental beam with polarization orthogonal to that of another fundamental beam; said two fundamental laser beams are well-collimated for minimizing optical loss and noise; said two fundamental laser beams are combined in the optically nonlinear frequency mixing crystal due to walk-off effect in a beam-combining birefringent crystal; microchip lasers can be made at high volume by growing the crystals into layers with uniform coatings at appropriate wavelengths and cutting the integrated crystal into small pieces.
 17. A monolithic microchip laser as of claim 11, wherein: said resonant cavities are composed of mirrors highly reflective at the desired wavelengths to circulate the power, while highly transmissive at undesired wavelengths to suppress the lasing; output coupler is highly transmissive to the wavelength at which the laser output produces; one or two concave mirrors are optionally used for supporting laser oscillation of high power and/or for reducing beam divergence induced optical loss/noise; said concave mirrors can be separated from the microchip laser or optically bonded for monolithic structure; one mirror can be shared by two or more cavities, however, each cavity supports only one laser oscillation; mirrors and interfacial coatings are so designed as to maximize the frequency conversion efficiency and to minimize intracavity optical loss.
 18. An apparatus of compact size for efficient frequency conversion comprising: two light sources for generating two monochromatic lights at the fundamental wavelengths with mutually orthogonal polarizations; a birefringent crystal for spatially combining two orthogonally polarized fundamental laser beams via walk-off effect; a nonlinear optical crystal for sum frequency or difference frequency generation; an optional thermoelectric controller for automatic temperature control of pump diodes and/or laser device; an algorithm for automatic power control and optimization of SFM or DFM efficiency; optical elements optional according to specific applications; wherein: said monochromatic lights at the fundamental wavelengths are generated in separate cavities; said monochromatic lights at fundamental wavelengths are changeable in intensities, wavelengths, operation modes, and propagation directions; said light sources are selected according to specific features including wavelength, polarization, spectral width, operation modes, and others to be carried into the laser output; interaction of said monochromatic lights at fundamental wavelengths with a nonlinear optical crystal produces a new light at the sum frequency or difference frequency; fulfillment of phase matching conditions is achieved by appropriate selection of birefringent material, its geometry, and/or temperature.
 19. An apparatus as of claim 18, wherein: said birefringent crystal is an un-doped YVO₄ or TiO₂ or the like; said nonlinear crystal is KTP or the like and its exterior surface is coated highly transmissive or anti-reflective to 491 nm; the first light source further comprises: an a-cut Nd:YVO₄ laser gain medium with C-axis parallel to the principal plane of said birefringent crystal; a pump light at 808 nm with π polarization for end pumping; and a resonant cavity to support laser oscillation at 1064 nm; the second light source further comprises: an a-cut Nd:YVO₄ laser gain medium with C-axis normal to the principal plane of said birefringent crystal; a pump light at 808 nm with π polarization for end pumping; and a resonant cavity to support laser oscillation at 914 nm and to suppress oscillation at 1064 nm and 1342 nm; said pump lights can be produced from two laser diodes, separated or packaged together, or from a dual-emitter, or emitted from a single laser diode and split into two components in a birefringent crystal; a concave mirror is optionally used for improving beam collimation.
 20. An apparatus as of claim 18, wherein: said birefringent crystal is an un-doped YVO₄ or TiO₂ or the like: said nonlinear crystal is KTP or the like and its exterior surface is coated highly transmissive or anti-reflective to 532 nm; said light sources are composed of: an Nd:YAG laser gain medium; two pump lights at 808 nm separated with a predefined distance for end pumping; and two independent resonant cavities to support two independent laser oscillations of orthogonal polarizations, both at 1064 nm; wherein: said two bump lights are generated from a dual-emitter, or a single laser diode, together with a birefringent crystal, which is optically bonded to the laser gain medium for separating the p-component and s-component, and a set of optical lenses placed between said laser diode and said birefringent crystal for beam shaping; a concave mirror is optionally used for improving beam collimation. 